1. Field of the Invention
The present invention relates to a method for determining geometrical-optical aberrations up to and including 3rd order in particle-optical, probe-forming systems, in particular scanning electron microscopes comprising an essentially punctiform source, which emits the particles, lenses for influencing the particle beam, an object, which is imaged by the particles, and a detector for registering the particles or imaging the object, the object being imaged by a particle beam focussed on the object, and the image being recorded; the process being repeated with an overfocussed and an underfocussed beam, which produce the images (overfocussed) and (underfocussed), the images being transformed in Fourier space, the transformation of the overfocussed image being divided by that of the transformed focussed image, and the quotient being obtained, and the transformation of the underfocussed image being divided by that of the transformed focussed image and the quotient being obtained.
2. Description of the Background Art
Scanning electron microscopes operate according to the principle that a sharply focussed electron beam, whose diameter determines the efficiency and resolution, is guided line by line over an object surface to be analyzed. The electrons passing through the object or scattered back therefrom, or the secondary electrons released in the object surface, are either collected in a collector or amplified by means of a scintillator and a downstream photomultiplier and used for controlling the display. For emission of the electrons, a source under high voltage is used, which usually takes the form of a tungsten tip, whose diameter is of the order of a few nm. Via the tip, an essentially punctiform particle source can be provided with virtually any accuracy. The image of the source, that is to say the tungsten tip, of the microscope optics is usually described as the probe.
In the case of particle optical systems, in particular scanning electron microscopes, the resolution capacity and the quality of the image is limited, inter alia, by the geometrical-optical aberrations, which have the consequence that punctiform objects are not reproduced in an ideally punctiform manner in the image. In the vicinity of the theoretical image point, the caustic is produced as the envelope of the rays actually intersecting in its vicinity. Spherical aberration is known, in which the axially parallel incident rays intersect in the image space respectively before or after the image point supplied by the paraxial rays. The axial image aberrations of higher “foldedness” lead to enlargements of the image point, which will be different depending on the azimuth. In the case of two-fold astigmatism, a circular object in the image plane is distorted into an elliptical image, since the meriodonal and sagittal rays perpendicular thereto have different focal lengths. For correction of these axial imaging aberrations extending up to a 3rd order, it is known to use correctives consisting of non-circular lens systems, as well as, for example from PCT/DE98/02596, a method for eliminating all axial image aberrations up to the 3rd order, in order to increase the resolution capacity.
The article in Japanese Journal Appl. Phys., Volume 38 (1999), pages 957ff, and GB 2 305 324 A disclose methods for determining 1st order image aberrations, in which the images from two different focussings are transformed in Fourier space and, by forming the quotient, are used for determining the image aberration coefficients. The method described here, however, is unsuitable for determining higher order image aberrations.
A disadvantage of this can be seen in the fact that the information obtained in the image point is determined both by the optical image aberrations of the imaging, probe-forming optical system and by the object structure itself. For determination of the image aberration, it would therefore be necessary to know the object structure, in order, from the image obtained, and the known object structure to be able to draw conclusions about the nature and size of the image aberration.